1. Field of the Invention
This invention relates to a control system for internal combustion engines, which detects the combustion state of the engine and controls the amount of fuel supplied to the engine based on the detected combustion state.
2. Prior Art
Conventionally, it is widely known to control the air-fuel ratio of a mixture supplied to an internal combustion engine to a leaner value than a stoichiometric air-fuel ratio in order to curtail the fuel consumption, as well as to recirculate part of exhaust gases from the exhaust system of an internal combustion engine to the intake system of the same in order to improve exhaust emission characteristics of the engine. However, if the leaning of the air-fuel ratio and/or the exhaust gas recirculation is carried out to an excessive extent, the engine will have unstable combustion and hence have degraded driveability. To avoid this inconvenience, it has been conventionally proposed by Japanese Laid-Open Patent Publication (Kokai) No. 58-182516, to detect the combustion state of an internal combustion engine by the use of a vibration sensor, and enrich the air-fuel ratio of a mixture supplied to the engine when the detected vibration value from the vibration sensor exceeds a predetermined reference value.
According to the proposed method, however, the predetermined reference value employed to determine whether enriching of the air-fuel ratio is to be carried out is a fixed value. Therefore, if the predetermined reference value is set to a too large value, the combustion state of the engine can become unstable, causing degraded driveability, even when the detected vibration value is below the predetermined reference value, depending on manufacturing variations of component parts of the engine between production lots and/or the degree of deterioration or aging of component parts of the engine. To avoid such an inconvenience, it is required to set the predetermined reference value to a value which is considerably smaller than a limit value at or below which leaning of the air-fuel ratio is permitted. Thus, there is still a demand for a technique of controlling the fuel supply amount which can further improve the fuel consumption characteristic or fuel economy of an internal combustion engine, while maintaining good driveability of the engine.